Fuel cells and particularly polymer electrolyte membrane (“PEM”) fuel cells are actively under development by a large number of companies. These devices, while offering efficiency and environmental advantages, are too expensive at current prices to have a major market impact. Consequently, there is a world-wide effort to reduce the cost of these units.
Fuel cells for stationary applications are fueled primarily by methane and propane, from which hydrogen is obtained in a fuel processing unit that combines steam reforming with water-gas shifting and carbon monoxide cleanup. It is widely recognized that even 50 ppm of carbon monoxide (CO) in the fuel can coat the anode of the fuel cell, reducing the area available for hydrogen to react, and limiting the fuel cell current. CO is also a major poison with reformed methanol and direct methanol fuel cells.
Reforming methane produces about 10% or higher CO. This is reduced to about 1 percent CO in a water-gas shift reactor, followed by a reduction to 10 to 50 ppm in a CO clean-up reactor. Both the water-gas shift reactor and the clean-up reactor are major costs in the fuel cell system. For instance, in one approach, the PROX clean-up reactor uses two to three reaction stages operating at temperature of 160° C. to 190° C. compared to the stack temperature of 80° C. The water-gas shift reactor typically consists of two reactor stages operating at higher and lower temperatures. In addition, a stack running on 10 to 50 ppm of CO must be about twice the electrode area of a stack operating on pure H2.
Cleaning an anode of an electrochemical energy converter by changing the potential of the anode was proposed by Bockris in “Basis of Possible Continuous Self Activation In an Electrochemical Energy Converter”, J. Electroanal. Chem., vol. 7, pp. 487-490 (1964). In his scheme, a cleaning current pulse of about 40 mA was used. During the time the pulse was on, cleaning was accomplished but little or no power was produced. When the pulse was off, power was produced using the cleaned electrode, which gradually became re-covered with CO. Consequently, this system is most attractive when the cleaning pulses are of short duration in the duty cycle. The cleaning pulses may consume energy, so the power produced must be larger than the power consumed by the cleaning pulses for a net gain in power to be realized.
Publications using and extending this approach have appeared, including International Publication No. WO 98/42038 by Stimming et al. applying this technology to PEM fuel cells, and Carrette, Friedrich, Huber and Stimming, “Improvement of CO Tolerance of Proton Exchange Membrane Fuel Cells by a Pulsing Technique”, PCCP, v. 3, n. 3, Feb. 7, 2001, pp 320-324. The Stimming approach also used a cleaning current pulse of between 100 and 640 mA/cm2 with varying pulse durations and frequencies. Square wave current pulses, similar to the work of Bockris, are used. In addition, Stimming has proposed using positive voltage pulses for cleaning. Stimming showed that this method could clean electrodes with 1 percent CO in the feed stream for laboratory, bench-top experiments.
Wang and Fedkiw, “Pulsed-Potential Oxidation of Methanol, I”, J. Electrochem. Soc., v. 139 n. 9, September 1992, 2519-2525, and “Pulsed-Potential Oxidation of Methanol, II”, v. 139, n. 11, 3151-3158, showed that pulsing a direct methanol fuel cell with positive square wave pulses of a certain frequency could result in a substantial increase in output current. The increase was attributed to cleaning intermediates from the electrode.
The pulsing approaches used in the current patent and technical literature do not address pulsing waveform shapes other than square waves. In addition, methods of determining suitable waveform shapes for different electrodes, electrolytes, load characteristics, and operating conditions are not discussed. More powerful techniques are needed for electrode cleaning in fuel cells, particularly techniques that would allow the fuel cell to consistently and robustly operate on 1 percent and higher levels of CO, while eliminating the clean-up reactor, simplifying the reformer and shift reactors, and reducing the stack size. The invention reported herein utilizes the inherent dynamical properties of the electrode to improve the fuel cell performance and arrive at a suitable pulsing waveform shape or electrode voltage control method.
Furthermore, the literature to date that is known to us is restricted to CO levels less than 1 percent. The invention reported herein allows operation at higher levels of CO, which enables the reformer to be substantially simplified.